We propose to continue with work presently underway which involves the actions of singlet molecular oxygen on cholesterol, on 5 gamma-cholest-7-en-3 beta-ol, and on 5 alpha-lanost-8-en-3 beta-ol. With singlet oxygen generated under a variety of conditions we seek to detect and identify all major primary sterol hydroperoxide products formed with these three naturally occurring sterols as substrates. We presently seek to emphasize the reaction between 5 alpha-lanost-8-en-3 beta-ol and singlet oxygen, for which virtually nothing sound is known except what we have been able to accomplish. We seek to isolate the five or so products, purify and characterize them, and confirm their structures by chemical synthesis or other means. We also seek to develop suitable microanalytical means for their detection at very low levels in tissues.